Figures

- A bubble-driven microfluidic transport element for bioengineering

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Fig. 1. Layout of the experiment. The piezoelectric transducer generates a standing ultrasound field in the cuvette, which directly excites microbubbles adsorbed at the bottom and indirectly leads to streaming flow around solid particles. An inverted microscope with phase-contrast capability observes the ensuing flow and the motion of suspended vesicles or cells from below, through the use of a high-speed camera.

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Fig. 2. Streaming field around a bubble adsorbed at a wall, obtained from the Stokes singularity theory (solid lines). The large arrow indicates the leading-order singularity representing the bubble: a point force oriented perpendicular to the wall. Some experimental trajectories of vesicles are overlaid onto the simulation, showing qualitative agreement.

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Fig. 3. Stokes flow of a point force singularity oriented parallel to the wall (arrow), positioned at p = (2a,0, a). This is the leading-order RNW (Rayleigh–Nyborg–Westervelt) streaming flow induced around a small particle at p by an oscillating bubble (dashed circle) located at b = (0, 0, a).

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Fig. 4. A streamline in the streaming flow induced by a bubble–particle doublet, viewed from the side (Upper) and the top (Lower). The arrow indicates the position p = (2a,0, hp) of the stokeslet parallel to the wall, where hp/a = 0.75 and the stokeslet strength is s = 0.15 times that of the vertical stokeslet induced by the bubble.

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Fig. 5. A bubble (large circular object; a ≈ 80 μm) and a quartz particle (small, bright object near the center; ap ≈ 20 μm) form a streaming doublet with L ≈ 105 μm. A vesicle (arrows; R ≈ 10 μm) follows the streamlines of the doublet flow and is ruptured upon close encounter with the particle (Right). With these parameters, we infer the ratio of particle stokeslet and bubble stokeslet strengths to be s ≈ 0.15. This number, as well as the estimated standoff distance hp/a ≈ 0.75, was used for the simulation in Fig. 4. The images were taken at t = 0, 0.3, and 0.5 sec; taking into account the oscillatory character of the trajectory (Fig. 6), an average transport speed of ≈1 mm/sec results.

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Fig. 6. Top view of experimental trajectories of vesicles in the streaming flow of the bubble–particle doublet of Fig. 5. A small vesicle is directionally transported beyond the particle (dashed line trajectory). A larger vesicle (continuous line trajectory), that displayed in Fig. 5, is ruptured near the position of the particle (cross).

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Fig. 7. Streamline in the flow induced by a succession of two aligned doublets, viewed from the side (Upper) and the top (Lower). Both doublets are of the same type as that in Fig. 4.

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Fig. 8. Schematic of structural elements for a bubble microfluidics device. A hydrophilic substrate is etched away to leave small, ≈10- to 50-μm protrusions. Hydrophobic patches with a diameter of ≈10–100 μm (e.g., gold or Teflon) are then deposited next to the protrusions to accommodate the microbubbles and form a doublet.